The ability to increase the rate of growth and efficiency of food utilization of female animals to levels which correspond to those of normal intact male animals would have a significant impact on meat production. Attempts have been made to improve post-natal female growth by pre-natal manipulation of sexual differentiation. Pre-natal treatment with testosterone propionate followed by post-natal treatment with testosterone propionate and estradiol benzoate of heifer calves produced an additive improvement in growth rate, feed efficiency and carcass merit of heifers, DeHaan et al., J. Anim. Sci. 68: 2198-2207 (1990). Ewes exposed to testosterone between days 30 and 80, 50 and 100 or 70 and 120 of fetal life failed to show regular overt oestrous cycles, although some of the ewes ovulated. Prenatal androgenization, treatment with testosterone, has been shown to enhance ewe production efficiency by improving growth rate, feed efficiency and carcass merit, DeHaan et al., J. Amin. Sci. J. Anim. Sci. 65 (suppl. 1): 85 (1987). In utero treatment with testosterone propionate resulted in ewes with increased weight gain and feed efficiency along with lower fat, Jenkins et al., J. Anim. Sci. (suppl. 1) 65: 248 (1987). Exposure to testosterone propionate in utero resulted in gilts gaining weight at a higher level between day 28 and 125 but did not shown an increase in feed efficiency, Matulis et al., J. Anim. Sci. 65 (suppl. 1): 249 (1987). Female rats treated perinatally with testosterone propionate exhibited increased growth and efficiency of food utilization when compared to non-treated controls, Perry et al., J. Endocr. 81: 35-48 (1979).
The ability of males to grow at a rate higher than females generally becomes more evident at or after puberty suggesting that the difference may be due, at least in part, to the influence of sex steroids, Jansson et al., Endocrin. 114: 1287-1294 (1984). Gonadectomy of male and female rats results in a decrease in male growth and an increase in the growth of females, Weidemann, In: Daughaday (ed) Endocrine Control of Growth. Elsevier, New York, p. 67 (1981). Circulating estrogen suppresses somatic growth while testosterone is required for male type growth, Jansson et al., Endocrin. 114: 1287-1294 (1984). The sites of action on growth appears to be different for estrogen and for androgen, Jansson et al., Am. J. Physiol. 244: Ei35-Ei140 (1983).